Devices of this kind are known in practice and are represented, for example, on page 9 of the technical supplement RD93010/03.87 of the firm Hydromatik GmbH under the title Cross-Sensing. The power control of this known device is a summation power control which distributes the total drive power made available by the drive motor in a manner between the two variable displacement pumps in a demand oriented manner. Like every power control it is based on the principle of adjusting the displacement of the variable displacement pumps in dependence upon the first control pressure along a hyperbolically extending characteristic curve, so that the maximum drive torque or, with constant drive speed, the maximum power take up of the variable displacement pumps remains substantially constant over the entire operating region.
The spring arrangements associated with the power control valves of the known device consist in each case of a pressure spring, the setting value of which determines the backpressure and thus the maximum power take up of the respective variable displacement pump. The backpressure is chosen such that each variable displacement pump is set to 100% total drive power. This power can be transmitted by each variable displacement pump for as long as the pressure difference acting on the associated power control valve equals the set backpressure, i.e. there is no second control pressure, since the respective other variable displacement pump does not require any power.
With increasing power take up of the lastmentioned variable displacement pump, as a consequence of the appropriately rising second control pressure the pressure difference at the power control valve of the first variable displacement pump reduces and thus its power setting reduces.
However, as soon as both variable displacement pumps transmit in each case 50% of the total drive power, their power take up is restricted to this value. For this purpose an appropriate power restriction in the form of throttle and valve devices is provided, which prevents a reduction of the power setting of the respective other variable displacement pump. These devices are comparatively costly in terms of construction and are not a standard delivery component of the known device. They are installed subsequently by the customer during installation of the power control device and the variable displacement pumps in vehicles etc..